FIG. 1 shows a film thickness measuring device which is proposed in a co-pending U.S. patent application Ser. No. 886,782 filed on July 18, 1986 in which priority is claimed based on Japanese Patent Application No. 174609/1985 filed on Aug. 6, 1985.
In FIG. 1, reference numeral 1 designates a rotary shaft which is rotated at a predetermined speed; 2, a light shielding board which is disposed in parallel with the rotary shaft 1 with a predetermined distance therebetween ; and 3, a sheet including a sheet member and a film formed thereon having a predetermined thickness, the thickness of the film being to be measured. The sheet 3 is conveyed at a speed equal to the rotation speed of the rotarys shaft 1 while being in close contact with the latter 1.
Further in FIG. 1, laser beam generators 4 and 5 are arranged at predetermined angles with respect to each other, for generating laser beams 4a and 5a, respectively, A reflecting mirror 6 is provided to cause the laser beam 4a to scan a gap A shown in FIG. 2 between the surface of the rotary shaft 1 and the light shielding board 2. Further, the reflecting mirror 6 also causes the laser beam 5a to scan a gap B shown in FIG. 2 between the sheet 3 under measurement and the light shielding board 2. Reference numerals 7 and 8 designate lenses for converging the laser beams 4a and 5a, respectively, which have been reflected by the reflecting mirror 6; 9 and 10, lenses for converging the laser beams 4a and 5a which have scanned the gaps A and B, respectively; 11 and 12, light receiving units; 13 and 14, counters; 15, an arithmetic unit; and 16, a display unit.
The film thickness measuring device thus constructed operates as follows:
The laser beams 4a and 5a generated by the laser beam generators 4 and 5 are directed to the reflecting mirror 6, so that they are caused to scan the respective gaps A and B at the same angular velocity. The laser beams 4a and 5a reflected by the mirror 6 are converged by the lenses 7 and 8 so that they are made minimum in beam diameter at the gaps A and B, respectively, and are run in a direction perpendicular to the rotary shaft 1; i.e., in the direction of the gaps at the predetermined speed. In this operation, the light receiving units 11 and 12 receive the laser beams 4a and 5a which have passed through the gaps A and B, respectively. Accordingly, the output signals of the light receiving units 11 and 12 are pulse signals, the widths of which are proportional to the dimensions of the gaps A and B. The pulse signals are counted by the counters 13 and 14, the counted values of which are applied to the arithmetic unit 15 where the thickness is calculated using the counted values. The thickness thus calculated is displayed on the display unit 16.
The thickness t.sub.x of the sheet member 3 under measurement can be obtained from the following equation (1): EQU t.sub.x =t.sub.o (1-b/a) (1)
where a is the counted value of the counter 13, b is the counted value of the counter 14, and t.sub.o is the dimension of the gap A which has been set.
The film thickness t can be obtained by subtracting from the thickness t.sub.x of the sheet 3 thus calculated the thickness of the sheet member which has been known.
As was described above, the film thickness measuring device shown in FIG. 1 measures the thickness of the sheet member and that of the film formed thereon by referring to the dimension of the gap between the rotary shaft and the light shielding plate as a reference value. Therefore, the film thickness measuring device suffers from difficulties that, as the rotary shaft turns, the dimension of the gap varies with time because of the eccentricity of uneven surface of the rotary shaft and accordingly the measurement value also varies; that is, the measurement is not stable nor accurate.